Custom biocatalyst design for use in synthetic chemistry is a very hotly pursued area of biotechnological research. Not only are enzymes attractive because they achieve remarkable rate enhancements with little or no requirement for energy input, but they also avoid the toxic waste problem associated with most conventional catalysts. Furthermore, the precision with which enzymes act, both in terms of regio- and stereo-specificity, is of substantial interest to pharmaceutical and fine-chemical manufactures. Progress in directed evolution approaches to custom biocatalyst design has been brisk, especially in the past five years. However, many potentially very useful alterations to enzyme function are inaccessible because no effective technology exists for reshaping gross features of the active site. Current directed evolution methodologies excel in the specificity (i.e. focusing selective pressure precisely on the sought functionality) with which they search regions of sequence space proximal to the starting structure, but to make substantial changes to binding pocket geometry it is necessary to first sample a vast area of sequence space in the hope of identifying reasonable starting points for optimization. Success in this crucial first step is dependent upon two factors: library size and threshold of selection. The proposed technology utilizes an innovative combinatorial approach, based on pairing of complementary protein fragments, to access libraries of unprecedented size with an extremely low threshold for affinity selection. Furthermore, the proposed methodology obviates ligand immobilization and thus is fully compatible with in vitro evolution of the active site lids so often found to be critical for natural enzyme proficiency. [unreadable] [unreadable]